fs/netfs/fscache_volume.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-or-later
 /* Volume-level cache cookie handling.
  *
  * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
  * Written by David Howells (dhowells@redhat.com)
  */

 #define FSCACHE_DEBUG_LEVEL COOKIE
 #include <linux/export.h>
 #include <linux/slab.h>
 #include "internal.h"

 #define fscache_volume_hash_shift 10
 static struct hlist_bl_head fscache_volume_hash[1 << fscache_volume_hash_shift];
 static atomic_t fscache_volume_debug_id;
 static LIST_HEAD(fscache_volumes);

 static void fscache_create_volume_work(struct work_struct *work);

 struct fscache_volume *fscache_get_volume(struct fscache_volume *volume,
 					  enum fscache_volume_trace where)
 {
 	int ref;

 	__refcount_inc(&volume->ref, &ref);
 	trace_fscache_volume(volume->debug_id, ref + 1, where);
 	return volume;
 }

 struct fscache_volume *fscache_try_get_volume(struct fscache_volume *volume,
 					      enum fscache_volume_trace where)
 {
 	int ref;

 	if (!__refcount_inc_not_zero(&volume->ref, &ref))
 		return NULL;

 	trace_fscache_volume(volume->debug_id, ref + 1, where);
 	return volume;
 }
 EXPORT_SYMBOL(fscache_try_get_volume);

 static void fscache_see_volume(struct fscache_volume *volume,
 			       enum fscache_volume_trace where)
 {
 	int ref = refcount_read(&volume->ref);

 	trace_fscache_volume(volume->debug_id, ref, where);
 }

 /*
  * Pin the cache behind a volume so that we can access it.
  */
 static void __fscache_begin_volume_access(struct fscache_volume *volume,
 					  struct fscache_cookie *cookie,
 					  enum fscache_access_trace why)
 {
 	int n_accesses;

 	n_accesses = atomic_inc_return(&volume->n_accesses);
 	smp_mb__after_atomic();
 	trace_fscache_access_volume(volume->debug_id, cookie ? cookie->debug_id : 0,
 				    refcount_read(&volume->ref),
 				    n_accesses, why);
 }

 /**
  * fscache_begin_volume_access - Pin a cache so a volume can be accessed
  * @volume: The volume cookie
  * @cookie: A datafile cookie for a tracing reference (or NULL)
  * @why: An indication of the circumstances of the access for tracing
  *
  * Attempt to pin the cache to prevent it from going away whilst we're
  * accessing a volume and returns true if successful.  This works as follows:
  *
  *  (1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
  *      then we return false to indicate access was not permitted.
  *
  *  (2) If the cache tests as live, then we increment the volume's n_accesses
  *      count and then recheck the cache liveness, ending the access if it
  *      ceased to be live.
  *
  *  (3) When we end the access, we decrement the volume's n_accesses and wake
  *      up the any waiters if it reaches 0.
  *
  *  (4) Whilst the cache is caching, the volume's n_accesses is kept
  *      artificially incremented to prevent wakeups from happening.
  *
  *  (5) When the cache is taken offline, the state is changed to prevent new
  *      accesses, the volume's n_accesses is decremented and we wait for it to
  *      become 0.
  *
  * The datafile @cookie and the @why indicator are merely provided for tracing
  * purposes.
  */
 bool fscache_begin_volume_access(struct fscache_volume *volume,
 				 struct fscache_cookie *cookie,
 				 enum fscache_access_trace why)
 {
 	if (!fscache_cache_is_live(volume->cache))
 		return false;
 	__fscache_begin_volume_access(volume, cookie, why);
 	if (!fscache_cache_is_live(volume->cache)) {
 		fscache_end_volume_access(volume, cookie, fscache_access_unlive);
 		return false;
 	}
 	return true;
 }

 /**
  * fscache_end_volume_access - Unpin a cache at the end of an access.
  * @volume: The volume cookie
  * @cookie: A datafile cookie for a tracing reference (or NULL)
  * @why: An indication of the circumstances of the access for tracing
  *
  * Unpin a cache volume after we've accessed it.  The datafile @cookie and the
  * @why indicator are merely provided for tracing purposes.
  */
 void fscache_end_volume_access(struct fscache_volume *volume,
 			       struct fscache_cookie *cookie,
 			       enum fscache_access_trace why)
 {
 	int n_accesses;

 	smp_mb__before_atomic();
 	n_accesses = atomic_dec_return(&volume->n_accesses);
 	trace_fscache_access_volume(volume->debug_id, cookie ? cookie->debug_id : 0,
 				    refcount_read(&volume->ref),
 				    n_accesses, why);
 	if (n_accesses == 0)
 		wake_up_var(&volume->n_accesses);
 }
 EXPORT_SYMBOL(fscache_end_volume_access);

 static bool fscache_volume_same(const struct fscache_volume *a,
 				const struct fscache_volume *b)
 {
 	size_t klen;

 	if (a->key_hash	!= b->key_hash ||
 	    a->cache	!= b->cache ||
 	    a->key[0]	!= b->key[0])
 		return false;

 	klen = round_up(a->key[0] + 1, sizeof(__le32));
 	return memcmp(a->key, b->key, klen) == 0;
 }

 static bool fscache_is_acquire_pending(struct fscache_volume *volume)
 {
 	return test_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &volume->flags);
 }

 static void fscache_wait_on_volume_collision(struct fscache_volume *candidate,
 					     unsigned int collidee_debug_id)
 {
 	wait_on_bit_timeout(&candidate->flags, FSCACHE_VOLUME_ACQUIRE_PENDING,
 			    TASK_UNINTERRUPTIBLE, 20 * HZ);
 	if (fscache_is_acquire_pending(candidate)) {
 		pr_notice("Potential volume collision new=%08x old=%08x",
 			  candidate->debug_id, collidee_debug_id);
 		fscache_stat(&fscache_n_volumes_collision);
 		wait_on_bit(&candidate->flags, FSCACHE_VOLUME_ACQUIRE_PENDING,
 			    TASK_UNINTERRUPTIBLE);
 	}
 }

 /*
  * Attempt to insert the new volume into the hash.  If there's a collision, we
  * wait for the old volume to complete if it's being relinquished and an error
  * otherwise.
  */
 static bool fscache_hash_volume(struct fscache_volume *candidate)
 {
 	struct fscache_volume *cursor;
 	struct hlist_bl_head *h;
 	struct hlist_bl_node *p;
 	unsigned int bucket, collidee_debug_id = 0;

 	bucket = candidate->key_hash & (ARRAY_SIZE(fscache_volume_hash) - 1);
 	h = &fscache_volume_hash[bucket];

 	hlist_bl_lock(h);
 	hlist_bl_for_each_entry(cursor, p, h, hash_link) {
 		if (fscache_volume_same(candidate, cursor)) {
 			if (!test_bit(FSCACHE_VOLUME_RELINQUISHED, &cursor->flags))
 				goto collision;
 			fscache_see_volume(cursor, fscache_volume_get_hash_collision);
 			set_bit(FSCACHE_VOLUME_COLLIDED_WITH, &cursor->flags);
 			set_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &candidate->flags);
 			collidee_debug_id = cursor->debug_id;
 			break;
 		}
 	}

 	hlist_bl_add_head(&candidate->hash_link, h);
 	hlist_bl_unlock(h);

 	if (fscache_is_acquire_pending(candidate))
 		fscache_wait_on_volume_collision(candidate, collidee_debug_id);
 	return true;

 collision:
 	fscache_see_volume(cursor, fscache_volume_collision);
 	hlist_bl_unlock(h);
 	return false;
 }

 /*
  * Allocate and initialise a volume representation cookie.
  */
 static struct fscache_volume *fscache_alloc_volume(const char *volume_key,
 						   const char *cache_name,
 						   const void *coherency_data,
 						   size_t coherency_len)
 {
 	struct fscache_volume *volume;
 	struct fscache_cache *cache;
 	size_t klen, hlen;
 	u8 *key;

 	klen = strlen(volume_key);
 	if (klen > NAME_MAX)
 		return NULL;

 	if (!coherency_data)
 		coherency_len = 0;

 	cache = fscache_lookup_cache(cache_name, false);
 	if (IS_ERR(cache))
 		return NULL;

 	volume = kzalloc(struct_size(volume, coherency, coherency_len),
 			 GFP_KERNEL);
 	if (!volume)
 		goto err_cache;

 	volume->cache = cache;
 	volume->coherency_len = coherency_len;
 	if (coherency_data)
 		memcpy(volume->coherency, coherency_data, coherency_len);
 	INIT_LIST_HEAD(&volume->proc_link);
 	INIT_WORK(&volume->work, fscache_create_volume_work);
 	refcount_set(&volume->ref, 1);
 	spin_lock_init(&volume->lock);

 	/* Stick the length on the front of the key and pad it out to make
 	 * hashing easier.
 	 */
 	hlen = round_up(1 + klen + 1, sizeof(__le32));
 	key = kzalloc(hlen, GFP_KERNEL);
 	if (!key)
 		goto err_vol;
 	key[0] = klen;
 	memcpy(key + 1, volume_key, klen);

 	volume->key = key;
 	volume->key_hash = fscache_hash(0, key, hlen);

 	volume->debug_id = atomic_inc_return(&fscache_volume_debug_id);
 	down_write(&fscache_addremove_sem);
 	atomic_inc(&cache->n_volumes);
 	list_add_tail(&volume->proc_link, &fscache_volumes);
 	fscache_see_volume(volume, fscache_volume_new_acquire);
 	fscache_stat(&fscache_n_volumes);
 	up_write(&fscache_addremove_sem);
 	_leave(" = v=%x", volume->debug_id);
 	return volume;

 err_vol:
 	kfree(volume);
 err_cache:
 	fscache_put_cache(cache, fscache_cache_put_alloc_volume);
 	fscache_stat(&fscache_n_volumes_nomem);
 	return NULL;
 }

 /*
  * Create a volume's representation on disk.  Have a volume ref and a cache
  * access we have to release.
  */
 static void fscache_create_volume_work(struct work_struct *work)
 {
 	const struct fscache_cache_ops *ops;
 	struct fscache_volume *volume =
 		container_of(work, struct fscache_volume, work);

 	fscache_see_volume(volume, fscache_volume_see_create_work);

 	ops = volume->cache->ops;
 	if (ops->acquire_volume)
 		ops->acquire_volume(volume);
 	fscache_end_cache_access(volume->cache,
 				 fscache_access_acquire_volume_end);

 	clear_and_wake_up_bit(FSCACHE_VOLUME_CREATING, &volume->flags);
 	fscache_put_volume(volume, fscache_volume_put_create_work);
 }

 /*
  * Dispatch a worker thread to create a volume's representation on disk.
  */
 void fscache_create_volume(struct fscache_volume *volume, bool wait)
 {
 	if (test_and_set_bit(FSCACHE_VOLUME_CREATING, &volume->flags))
 		goto maybe_wait;
 	if (volume->cache_priv)
 		goto no_wait; /* We raced */
 	if (!fscache_begin_cache_access(volume->cache,
 					fscache_access_acquire_volume))
 		goto no_wait;

 	fscache_get_volume(volume, fscache_volume_get_create_work);
 	if (!schedule_work(&volume->work))
 		fscache_put_volume(volume, fscache_volume_put_create_work);

 maybe_wait:
 	if (wait) {
 		fscache_see_volume(volume, fscache_volume_wait_create_work);
 		wait_on_bit(&volume->flags, FSCACHE_VOLUME_CREATING,
 			    TASK_UNINTERRUPTIBLE);
 	}
 	return;
 no_wait:
 	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
 	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
 }

 /*
  * Acquire a volume representation cookie and link it to a (proposed) cache.
  */
 struct fscache_volume *__fscache_acquire_volume(const char *volume_key,
 						const char *cache_name,
 						const void *coherency_data,
 						size_t coherency_len)
 {
 	struct fscache_volume *volume;

 	volume = fscache_alloc_volume(volume_key, cache_name,
 				      coherency_data, coherency_len);
 	if (!volume)
 		return ERR_PTR(-ENOMEM);

 	if (!fscache_hash_volume(volume)) {
 		fscache_put_volume(volume, fscache_volume_put_hash_collision);
 		return ERR_PTR(-EBUSY);
 	}

 	fscache_create_volume(volume, false);
 	return volume;
 }
 EXPORT_SYMBOL(__fscache_acquire_volume);

 static void fscache_wake_pending_volume(struct fscache_volume *volume,
 					struct hlist_bl_head *h)
 {
 	struct fscache_volume *cursor;
 	struct hlist_bl_node *p;

 	hlist_bl_for_each_entry(cursor, p, h, hash_link) {
 		if (fscache_volume_same(cursor, volume)) {
 			fscache_see_volume(cursor, fscache_volume_see_hash_wake);
 			clear_and_wake_up_bit(FSCACHE_VOLUME_ACQUIRE_PENDING,
 					      &cursor->flags);
 			return;
 		}
 	}
 }

 /*
  * Remove a volume cookie from the hash table.
  */
 static void fscache_unhash_volume(struct fscache_volume *volume)
 {
 	struct hlist_bl_head *h;
 	unsigned int bucket;

 	bucket = volume->key_hash & (ARRAY_SIZE(fscache_volume_hash) - 1);
 	h = &fscache_volume_hash[bucket];

 	hlist_bl_lock(h);
 	hlist_bl_del(&volume->hash_link);
 	if (test_bit(FSCACHE_VOLUME_COLLIDED_WITH, &volume->flags))
 		fscache_wake_pending_volume(volume, h);
 	hlist_bl_unlock(h);
 }

 /*
  * Drop a cache's volume attachments.
  */
 static void fscache_free_volume(struct fscache_volume *volume)
 {
 	struct fscache_cache *cache = volume->cache;

 	if (volume->cache_priv) {
 		__fscache_begin_volume_access(volume, NULL,
 					      fscache_access_relinquish_volume);
 		if (volume->cache_priv)
 			cache->ops->free_volume(volume);
 		fscache_end_volume_access(volume, NULL,
 					  fscache_access_relinquish_volume_end);
 	}

 	down_write(&fscache_addremove_sem);
 	list_del_init(&volume->proc_link);
 	atomic_dec(&volume->cache->n_volumes);
 	up_write(&fscache_addremove_sem);

 	if (!hlist_bl_unhashed(&volume->hash_link))
 		fscache_unhash_volume(volume);

 	trace_fscache_volume(volume->debug_id, 0, fscache_volume_free);
 	kfree(volume->key);
 	kfree(volume);
 	fscache_stat_d(&fscache_n_volumes);
 	fscache_put_cache(cache, fscache_cache_put_volume);
 }

 /*
  * Drop a reference to a volume cookie.
  */
 void fscache_put_volume(struct fscache_volume *volume,
 			enum fscache_volume_trace where)
 {
 	if (volume) {
 		unsigned int debug_id = volume->debug_id;
 		bool zero;
 		int ref;

 		zero = __refcount_dec_and_test(&volume->ref, &ref);
 		trace_fscache_volume(debug_id, ref - 1, where);
 		if (zero)
 			fscache_free_volume(volume);
 	}
 }
 EXPORT_SYMBOL(fscache_put_volume);

 /*
  * Relinquish a volume representation cookie.
  */
 void __fscache_relinquish_volume(struct fscache_volume *volume,
 				 const void *coherency_data,
 				 bool invalidate)
 {
 	if (WARN_ON(test_and_set_bit(FSCACHE_VOLUME_RELINQUISHED, &volume->flags)))
 		return;

 	if (invalidate) {
 		set_bit(FSCACHE_VOLUME_INVALIDATE, &volume->flags);
 	} else if (coherency_data) {
 		memcpy(volume->coherency, coherency_data, volume->coherency_len);
 	}

 	fscache_put_volume(volume, fscache_volume_put_relinquish);
 }
 EXPORT_SYMBOL(__fscache_relinquish_volume);

 /**
  * fscache_withdraw_volume - Withdraw a volume from being cached
  * @volume: Volume cookie
  *
  * Withdraw a cache volume from service, waiting for all accesses to complete
  * before returning.
  */
 void fscache_withdraw_volume(struct fscache_volume *volume)
 {
 	int n_accesses;

 	_debug("withdraw V=%x", volume->debug_id);

 	/* Allow wakeups on dec-to-0 */
 	n_accesses = atomic_dec_return(&volume->n_accesses);
 	trace_fscache_access_volume(volume->debug_id, 0,
 				    refcount_read(&volume->ref),
 				    n_accesses, fscache_access_cache_unpin);

 	wait_var_event(&volume->n_accesses,
 		       atomic_read(&volume->n_accesses) == 0);
 }
 EXPORT_SYMBOL(fscache_withdraw_volume);

 #ifdef CONFIG_PROC_FS
 /*
  * Generate a list of volumes in /proc/fs/fscache/volumes
  */
 static int fscache_volumes_seq_show(struct seq_file *m, void *v)
 {
 	struct fscache_volume *volume;

 	if (v == &fscache_volumes) {
 		seq_puts(m,
 			 "VOLUME   REF   nCOOK ACC FL CACHE           KEY\n"
 			 "======== ===== ===== === == =============== ================\n");
 		return 0;
 	}

 	volume = list_entry(v, struct fscache_volume, proc_link);
 	seq_printf(m,
 		   "%08x %5d %5d %3d %02lx %-15.15s %s\n",
 		   volume->debug_id,
 		   refcount_read(&volume->ref),
 		   atomic_read(&volume->n_cookies),
 		   atomic_read(&volume->n_accesses),
 		   volume->flags,
 		   volume->cache->name ?: "-",
 		   volume->key + 1);
 	return 0;
 }

 static void *fscache_volumes_seq_start(struct seq_file *m, loff_t *_pos)
 	__acquires(&fscache_addremove_sem)
 {
 	down_read(&fscache_addremove_sem);
 	return seq_list_start_head(&fscache_volumes, *_pos);
 }

 static void *fscache_volumes_seq_next(struct seq_file *m, void *v, loff_t *_pos)
 {
 	return seq_list_next(v, &fscache_volumes, _pos);
 }

 static void fscache_volumes_seq_stop(struct seq_file *m, void *v)
 	__releases(&fscache_addremove_sem)
 {
 	up_read(&fscache_addremove_sem);
 }

 const struct seq_operations fscache_volumes_seq_ops = {
 	.start  = fscache_volumes_seq_start,
 	.next   = fscache_volumes_seq_next,
 	.stop   = fscache_volumes_seq_stop,
 	.show   = fscache_volumes_seq_show,
 };
 #endif /* CONFIG_PROC_FS */
	// SPDX-License-Identifier: GPL-2.0-or-later
	/* Volume-level cache cookie handling.
	*
	* Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
	* Written by David Howells (dhowells@redhat.com)
	*/

	#define FSCACHE_DEBUG_LEVEL COOKIE
	#include <linux/export.h>
	#include <linux/slab.h>
	#include "internal.h"

	#define fscache_volume_hash_shift 10
	static struct hlist_bl_head fscache_volume_hash[1 << fscache_volume_hash_shift];
	static atomic_t fscache_volume_debug_id;
	static LIST_HEAD(fscache_volumes);

	static void fscache_create_volume_work(struct work_struct *work);

	struct fscache_volume fscache_get_volume(struct fscache_volume volume,
	enum fscache_volume_trace where)
	{
	int ref;

	__refcount_inc(&volume->ref, &ref);
	trace_fscache_volume(volume->debug_id, ref + 1, where);
	return volume;
	}

	struct fscache_volume fscache_try_get_volume(struct fscache_volume volume,
	enum fscache_volume_trace where)
	{
	int ref;

	if (!__refcount_inc_not_zero(&volume->ref, &ref))
	return NULL;

	trace_fscache_volume(volume->debug_id, ref + 1, where);
	return volume;
	}
	EXPORT_SYMBOL(fscache_try_get_volume);

	static void fscache_see_volume(struct fscache_volume *volume,
	enum fscache_volume_trace where)
	{
	int ref = refcount_read(&volume->ref);

	trace_fscache_volume(volume->debug_id, ref, where);
	}

	/*
	* Pin the cache behind a volume so that we can access it.
	*/
	static void __fscache_begin_volume_access(struct fscache_volume *volume,
	struct fscache_cookie *cookie,
	enum fscache_access_trace why)
	{
	int n_accesses;

	n_accesses = atomic_inc_return(&volume->n_accesses);
	smp_mb__after_atomic();
	trace_fscache_access_volume(volume->debug_id, cookie ? cookie->debug_id : 0,
	refcount_read(&volume->ref),
	n_accesses, why);
	}

	/**
	* fscache_begin_volume_access - Pin a cache so a volume can be accessed
	* @volume: The volume cookie
	* @cookie: A datafile cookie for a tracing reference (or NULL)
	* @why: An indication of the circumstances of the access for tracing
	*
	* Attempt to pin the cache to prevent it from going away whilst we're
	* accessing a volume and returns true if successful. This works as follows:
	*
	* (1) If the cache tests as not live (state is not FSCACHE_CACHE_IS_ACTIVE),
	* then we return false to indicate access was not permitted.
	*
	* (2) If the cache tests as live, then we increment the volume's n_accesses
	* count and then recheck the cache liveness, ending the access if it
	* ceased to be live.
	*
	* (3) When we end the access, we decrement the volume's n_accesses and wake
	* up the any waiters if it reaches 0.
	*
	* (4) Whilst the cache is caching, the volume's n_accesses is kept
	* artificially incremented to prevent wakeups from happening.
	*
	* (5) When the cache is taken offline, the state is changed to prevent new
	* accesses, the volume's n_accesses is decremented and we wait for it to
	* become 0.
	*
	* The datafile @cookie and the @why indicator are merely provided for tracing
	* purposes.
	*/
	bool fscache_begin_volume_access(struct fscache_volume *volume,
	struct fscache_cookie *cookie,
	enum fscache_access_trace why)
	{
	if (!fscache_cache_is_live(volume->cache))
	return false;
	__fscache_begin_volume_access(volume, cookie, why);
	if (!fscache_cache_is_live(volume->cache)) {
	fscache_end_volume_access(volume, cookie, fscache_access_unlive);
	return false;
	}
	return true;
	}

	/**
	* fscache_end_volume_access - Unpin a cache at the end of an access.
	* @volume: The volume cookie
	* @cookie: A datafile cookie for a tracing reference (or NULL)
	* @why: An indication of the circumstances of the access for tracing
	*
	* Unpin a cache volume after we've accessed it. The datafile @cookie and the
	* @why indicator are merely provided for tracing purposes.
	*/
	void fscache_end_volume_access(struct fscache_volume *volume,
	struct fscache_cookie *cookie,
	enum fscache_access_trace why)
	{
	int n_accesses;

	smp_mb__before_atomic();
	n_accesses = atomic_dec_return(&volume->n_accesses);
	trace_fscache_access_volume(volume->debug_id, cookie ? cookie->debug_id : 0,
	refcount_read(&volume->ref),
	n_accesses, why);
	if (n_accesses == 0)
	wake_up_var(&volume->n_accesses);
	}
	EXPORT_SYMBOL(fscache_end_volume_access);

	static bool fscache_volume_same(const struct fscache_volume *a,
	const struct fscache_volume *b)
	{
	size_t klen;

	if (a->key_hash != b->key_hash \|\|
	a->cache != b->cache \|\|
	a->key[0] != b->key[0])
	return false;

	klen = round_up(a->key[0] + 1, sizeof(__le32));
	return memcmp(a->key, b->key, klen) == 0;
	}

	static bool fscache_is_acquire_pending(struct fscache_volume *volume)
	{
	return test_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &volume->flags);
	}

	static void fscache_wait_on_volume_collision(struct fscache_volume *candidate,
	unsigned int collidee_debug_id)
	{
	wait_on_bit_timeout(&candidate->flags, FSCACHE_VOLUME_ACQUIRE_PENDING,
	TASK_UNINTERRUPTIBLE, 20 * HZ);
	if (fscache_is_acquire_pending(candidate)) {
	pr_notice("Potential volume collision new=%08x old=%08x",
	candidate->debug_id, collidee_debug_id);
	fscache_stat(&fscache_n_volumes_collision);
	wait_on_bit(&candidate->flags, FSCACHE_VOLUME_ACQUIRE_PENDING,
	TASK_UNINTERRUPTIBLE);
	}
	}

	/*
	* Attempt to insert the new volume into the hash. If there's a collision, we
	* wait for the old volume to complete if it's being relinquished and an error
	* otherwise.
	*/
	static bool fscache_hash_volume(struct fscache_volume *candidate)
	{
	struct fscache_volume *cursor;
	struct hlist_bl_head *h;
	struct hlist_bl_node *p;
	unsigned int bucket, collidee_debug_id = 0;

	bucket = candidate->key_hash & (ARRAY_SIZE(fscache_volume_hash) - 1);
	h = &fscache_volume_hash[bucket];

	hlist_bl_lock(h);
	hlist_bl_for_each_entry(cursor, p, h, hash_link) {
	if (fscache_volume_same(candidate, cursor)) {
	if (!test_bit(FSCACHE_VOLUME_RELINQUISHED, &cursor->flags))
	goto collision;
	fscache_see_volume(cursor, fscache_volume_get_hash_collision);
	set_bit(FSCACHE_VOLUME_COLLIDED_WITH, &cursor->flags);
	set_bit(FSCACHE_VOLUME_ACQUIRE_PENDING, &candidate->flags);
	collidee_debug_id = cursor->debug_id;
	break;
	}
	}

	hlist_bl_add_head(&candidate->hash_link, h);
	hlist_bl_unlock(h);

	if (fscache_is_acquire_pending(candidate))
	fscache_wait_on_volume_collision(candidate, collidee_debug_id);
	return true;

	collision:
	fscache_see_volume(cursor, fscache_volume_collision);
	hlist_bl_unlock(h);
	return false;
	}

	/*
	* Allocate and initialise a volume representation cookie.
	*/
	static struct fscache_volume fscache_alloc_volume(const char volume_key,
	const char *cache_name,
	const void *coherency_data,
	size_t coherency_len)
	{
	struct fscache_volume *volume;
	struct fscache_cache *cache;
	size_t klen, hlen;
	u8 *key;

	klen = strlen(volume_key);
	if (klen > NAME_MAX)
	return NULL;

	if (!coherency_data)
	coherency_len = 0;

	cache = fscache_lookup_cache(cache_name, false);
	if (IS_ERR(cache))
	return NULL;

	volume = kzalloc(struct_size(volume, coherency, coherency_len),
	GFP_KERNEL);
	if (!volume)
	goto err_cache;

	volume->cache = cache;
	volume->coherency_len = coherency_len;
	if (coherency_data)
	memcpy(volume->coherency, coherency_data, coherency_len);
	INIT_LIST_HEAD(&volume->proc_link);
	INIT_WORK(&volume->work, fscache_create_volume_work);
	refcount_set(&volume->ref, 1);
	spin_lock_init(&volume->lock);

	/* Stick the length on the front of the key and pad it out to make
	* hashing easier.
	*/
	hlen = round_up(1 + klen + 1, sizeof(__le32));
	key = kzalloc(hlen, GFP_KERNEL);
	if (!key)
	goto err_vol;
	key[0] = klen;
	memcpy(key + 1, volume_key, klen);

	volume->key = key;
	volume->key_hash = fscache_hash(0, key, hlen);

	volume->debug_id = atomic_inc_return(&fscache_volume_debug_id);
	down_write(&fscache_addremove_sem);
	atomic_inc(&cache->n_volumes);
	list_add_tail(&volume->proc_link, &fscache_volumes);
	fscache_see_volume(volume, fscache_volume_new_acquire);
	fscache_stat(&fscache_n_volumes);
	up_write(&fscache_addremove_sem);
	_leave(" = v=%x", volume->debug_id);
	return volume;

	err_vol:
	kfree(volume);
	err_cache:
	fscache_put_cache(cache, fscache_cache_put_alloc_volume);
	fscache_stat(&fscache_n_volumes_nomem);
	return NULL;
	}

	/*
	* Create a volume's representation on disk. Have a volume ref and a cache
	* access we have to release.
	*/
	static void fscache_create_volume_work(struct work_struct *work)
	{
	const struct fscache_cache_ops *ops;
	struct fscache_volume *volume =
	container_of(work, struct fscache_volume, work);

	fscache_see_volume(volume, fscache_volume_see_create_work);

	ops = volume->cache->ops;
	if (ops->acquire_volume)
	ops->acquire_volume(volume);
	fscache_end_cache_access(volume->cache,
	fscache_access_acquire_volume_end);

	clear_and_wake_up_bit(FSCACHE_VOLUME_CREATING, &volume->flags);
	fscache_put_volume(volume, fscache_volume_put_create_work);
	}

	/*
	* Dispatch a worker thread to create a volume's representation on disk.
	*/
	void fscache_create_volume(struct fscache_volume *volume, bool wait)
	{
	if (test_and_set_bit(FSCACHE_VOLUME_CREATING, &volume->flags))
	goto maybe_wait;
	if (volume->cache_priv)
	goto no_wait; /* We raced */
	if (!fscache_begin_cache_access(volume->cache,
	fscache_access_acquire_volume))
	goto no_wait;

	fscache_get_volume(volume, fscache_volume_get_create_work);
	if (!schedule_work(&volume->work))
	fscache_put_volume(volume, fscache_volume_put_create_work);

	maybe_wait:
	if (wait) {
	fscache_see_volume(volume, fscache_volume_wait_create_work);
	wait_on_bit(&volume->flags, FSCACHE_VOLUME_CREATING,
	TASK_UNINTERRUPTIBLE);
	}
	return;
	no_wait:
	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
	}

	/*
	* Acquire a volume representation cookie and link it to a (proposed) cache.
	*/
	struct fscache_volume __fscache_acquire_volume(const char volume_key,
	const char *cache_name,
	const void *coherency_data,
	size_t coherency_len)
	{
	struct fscache_volume *volume;

	volume = fscache_alloc_volume(volume_key, cache_name,
	coherency_data, coherency_len);
	if (!volume)
	return ERR_PTR(-ENOMEM);

	if (!fscache_hash_volume(volume)) {
	fscache_put_volume(volume, fscache_volume_put_hash_collision);
	return ERR_PTR(-EBUSY);
	}

	fscache_create_volume(volume, false);
	return volume;
	}
	EXPORT_SYMBOL(__fscache_acquire_volume);

	static void fscache_wake_pending_volume(struct fscache_volume *volume,
	struct hlist_bl_head *h)
	{
	struct fscache_volume *cursor;
	struct hlist_bl_node *p;

	hlist_bl_for_each_entry(cursor, p, h, hash_link) {
	if (fscache_volume_same(cursor, volume)) {
	fscache_see_volume(cursor, fscache_volume_see_hash_wake);
	clear_and_wake_up_bit(FSCACHE_VOLUME_ACQUIRE_PENDING,
	&cursor->flags);
	return;
	}
	}
	}

	/*
	* Remove a volume cookie from the hash table.
	*/
	static void fscache_unhash_volume(struct fscache_volume *volume)
	{
	struct hlist_bl_head *h;
	unsigned int bucket;

	bucket = volume->key_hash & (ARRAY_SIZE(fscache_volume_hash) - 1);
	h = &fscache_volume_hash[bucket];

	hlist_bl_lock(h);
	hlist_bl_del(&volume->hash_link);
	if (test_bit(FSCACHE_VOLUME_COLLIDED_WITH, &volume->flags))
	fscache_wake_pending_volume(volume, h);
	hlist_bl_unlock(h);
	}

	/*
	* Drop a cache's volume attachments.
	*/
	static void fscache_free_volume(struct fscache_volume *volume)
	{
	struct fscache_cache *cache = volume->cache;

	if (volume->cache_priv) {
	__fscache_begin_volume_access(volume, NULL,
	fscache_access_relinquish_volume);
	if (volume->cache_priv)
	cache->ops->free_volume(volume);
	fscache_end_volume_access(volume, NULL,
	fscache_access_relinquish_volume_end);
	}

	down_write(&fscache_addremove_sem);
	list_del_init(&volume->proc_link);
	atomic_dec(&volume->cache->n_volumes);
	up_write(&fscache_addremove_sem);

	if (!hlist_bl_unhashed(&volume->hash_link))
	fscache_unhash_volume(volume);

	trace_fscache_volume(volume->debug_id, 0, fscache_volume_free);
	kfree(volume->key);
	kfree(volume);
	fscache_stat_d(&fscache_n_volumes);
	fscache_put_cache(cache, fscache_cache_put_volume);
	}

	/*
	* Drop a reference to a volume cookie.
	*/
	void fscache_put_volume(struct fscache_volume *volume,
	enum fscache_volume_trace where)
	{
	if (volume) {
	unsigned int debug_id = volume->debug_id;
	bool zero;
	int ref;

	zero = __refcount_dec_and_test(&volume->ref, &ref);
	trace_fscache_volume(debug_id, ref - 1, where);
	if (zero)
	fscache_free_volume(volume);
	}
	}
	EXPORT_SYMBOL(fscache_put_volume);

	/*
	* Relinquish a volume representation cookie.
	*/
	void __fscache_relinquish_volume(struct fscache_volume *volume,
	const void *coherency_data,
	bool invalidate)
	{
	if (WARN_ON(test_and_set_bit(FSCACHE_VOLUME_RELINQUISHED, &volume->flags)))
	return;

	if (invalidate) {
	set_bit(FSCACHE_VOLUME_INVALIDATE, &volume->flags);
	} else if (coherency_data) {
	memcpy(volume->coherency, coherency_data, volume->coherency_len);
	}

	fscache_put_volume(volume, fscache_volume_put_relinquish);
	}
	EXPORT_SYMBOL(__fscache_relinquish_volume);

	/**
	* fscache_withdraw_volume - Withdraw a volume from being cached
	* @volume: Volume cookie
	*
	* Withdraw a cache volume from service, waiting for all accesses to complete
	* before returning.
	*/
	void fscache_withdraw_volume(struct fscache_volume *volume)
	{
	int n_accesses;

	_debug("withdraw V=%x", volume->debug_id);

	/* Allow wakeups on dec-to-0 */
	n_accesses = atomic_dec_return(&volume->n_accesses);
	trace_fscache_access_volume(volume->debug_id, 0,
	refcount_read(&volume->ref),
	n_accesses, fscache_access_cache_unpin);

	wait_var_event(&volume->n_accesses,
	atomic_read(&volume->n_accesses) == 0);
	}
	EXPORT_SYMBOL(fscache_withdraw_volume);

	#ifdef CONFIG_PROC_FS
	/*
	* Generate a list of volumes in /proc/fs/fscache/volumes
	*/
	static int fscache_volumes_seq_show(struct seq_file m, void v)
	{
	struct fscache_volume *volume;

	if (v == &fscache_volumes) {
	seq_puts(m,
	"VOLUME REF nCOOK ACC FL CACHE KEY\n"
	"======== ===== ===== === == =============== ================\n");
	return 0;
	}

	volume = list_entry(v, struct fscache_volume, proc_link);
	seq_printf(m,
	"%08x %5d %5d %3d %02lx %-15.15s %s\n",
	volume->debug_id,
	refcount_read(&volume->ref),
	atomic_read(&volume->n_cookies),
	atomic_read(&volume->n_accesses),
	volume->flags,
	volume->cache->name ?: "-",
	volume->key + 1);
	return 0;
	}

	static void fscache_volumes_seq_start(struct seq_file m, loff_t *_pos)
	__acquires(&fscache_addremove_sem)
	{
	down_read(&fscache_addremove_sem);
	return seq_list_start_head(&fscache_volumes, *_pos);
	}

	static void fscache_volumes_seq_next(struct seq_file m, void v, loff_t _pos)
	{
	return seq_list_next(v, &fscache_volumes, _pos);
	}

	static void fscache_volumes_seq_stop(struct seq_file m, void v)
	__releases(&fscache_addremove_sem)
	{
	up_read(&fscache_addremove_sem);
	}

	const struct seq_operations fscache_volumes_seq_ops = {
	.start = fscache_volumes_seq_start,
	.next = fscache_volumes_seq_next,
	.stop = fscache_volumes_seq_stop,
	.show = fscache_volumes_seq_show,
	};
	#endif /* CONFIG_PROC_FS */